<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<title></title>
	</head>
	
	<body>
		
	</body>
	
	<script>
		var a = null;  //空 不等于没有, 对象的一种值，叫空值
		console.log( typeof a );
		var b;
		console.log( typeof b );
	</script>
	
	
</html>



